Key points

Can be spread on low cost commodity hardware
Native sharding mechanism to build a fully resilient implementation without the need to add (costly) specific option to the stack.
Migration from RDBMS to MongoDB is possible by the means of tools, of course this means that the way the applications are working should be reviewed and adapted.
Some BI vendors provide native MongoDB connector, without using SQL, like MicroStrategy.
Write operations are ACID.
Uses write-ahead logging to an on-disk journal. Change operations are written first into the journal; in case of server failure or error before writing in the database, the journaled operations can be reapplied

Migration to MongoDB

Use the mongoimport tool.

Use any ETL tools (Informatica, Pentaho, Talend with native connector).
Many migrations involve running the existing RDBMS in parallel with the new MongoDB database, incrementally transferring production data:

As records are retrieved from the RDBMS, the application writes them back out to MongoDB in the required document schema.

Consistency checkers, for example using MD5 checksums, can be used to validate the migrated data.

All newly created or updated data is written to MongoDB only.

Shutterfly used this incremental approach to migrate the metadata of 6 billion images and 20 TB of data from Oracle to MongoDB.

Security in MongoDB

Require MongoDB Enterprise Advanced subscription to have all the advanced security features

Without subscription, only internal users authentication.
Key file to authenticate node at the cluster level. X.509 certificates can also be used.
User authentication inside MongoDB on a per database level

Using roles for fine-grained privileges for users or applications (at DB and collection level)

You can have privilege to insert data but not to update or delete

Privileges to create DB or collections

Cluster wide-privileges for some users

=> follow a bit the granularity of privileges in RDBMS, allowing segregation of duties

Possibility to restrict the returned content of a document (removing some fields) - done via Redaction stage of the Aggregation Pipeline
Audit the administrative action, schema action and data manipulation

Filtering of the logs is possible

Logs can be written to multiple location in a variety of format (JSON, BSON, Syslog, …)

Encryption

Of the communication links

Of the storage (encrypted storage engine since version 3.2)

Transparent for the application

Support KMIP for integration with third party key management appliance